An ophthalmoscope is used to observe the patient's eye background, for example for retinal examinations. An ophthalmoscope delivers high-resolution color or black-and-white images in continuous succession, so that it can be used to diagnose the eye as well as for carrying out and documenting therapeutic interventions. Imaging of the eye background, for example the retina, constitutes an optical challenge in this regard since both the illumination and the observation of the eye background is carried out through a comparatively small entry pupil of the eye. The eye background furthermore generally has only a weak reflectivity, which dominates for red color components, so that high-contrast color images of the eye background can in general be produced only by a light source with strong blue and green components.
Optical examination of the eye background with the aid of an illumination beam, which is examined as an observation beam after reflection for example from the retina, is moreover made difficult by undesired further reflections from the interfaces of the cornea as well as by undesired light scattering for example from vitreous body turbidities. All these perturbing light beams, which may be contained in the observation beam but are not attributable to the desired reflection of the illumination beam from the part of the eye to be examined, will be referred to together below as “stray light”.
In order to resolve this stray light problem, EP 1389943 B1 discloses an ophthalmoscope for examining a patient's eye, comprising, an illumination device for generating an illumination beam, illumination imaging optics for imaging the illumination beam onto the eye, and means for scanning the illumination beam over the eye, as well as an observation device which comprises an electronic sensor with an array of photosensitive pixels, which can respectively be activated and/or read out in rows, observation imaging optics for imaging an observation beam, generated by reflection of the illumination beam from the eye, onto the observation device, and means for excluding stray light from the observation beam.
The illumination device used in this case is particularly a halogen lamp, the light of which is collimated by means of a condenser and focused by the illumination imaging optics onto the patient's eye background. The observation beam reflected by the eye background is imaged by the observation imaging optics onto an image plane, in which there is a CCD sensor as an observation device.
The means for scanning the illumination beam over the eye in this ophthalmoscope according to the species are formed by a slit shutter, which oscillates in front of the halogen lamp in the illumination beam collimated by the condenser. This shutter is made of an opaque material, for example a flat metal material and transmits only a linear segment of the illumination beam, which is defined by the size of the slit shutter and is likewise scanned to and fro over the patient's eye at this oscillation frequency.
The means for excluding stray light from the observation beam in this ophthalmoscope according to the species are also formed by a mechanically oscillating slit shutter. In particular, EP 1389943 B1 proposes that the slit shutter oscillating in front of the halogen lamp and the slit shutter oscillating in front of the CCD sensor, which in any event must be synchronised with one another for imaging reasons, should be formed as a shutter slit pair in a common metal sheet.
In practice, however, it has been found that particularly the selection of a metal sheet which has a slit shutter and oscillates in front of the sensor leads to problems. This is because on the one hand the oscillations of the mechanical oscillator for the metal sheet can be decoupled only insufficiently from the housing of the instrument, which generally entails difficulties in handling the ophthalmoscope and in particular degradations of the image sharpness owing to co-vibration of the sensor.
On the other hand, in order to achieve a maximally compact configuration of the ophthalmoscope, the shutter in the observation beam must lie as accurately as possible in the image plane of the observation device and therefore on the sensor. It is nevertheless compulsory to maintain a certain minimum distance of the metal sheet oscillating from the sensor, which inevitably leads to inferior suppression of perturbing stray light.